Volatilization And Incorporation Of Heavy Metals During Portland Cement Clinker Production Using Sewage Sludge/Pb-Zn Slag

Co-processing of solid wastes in cement kiln is an important way of recycling, recovering and reusing wastes. The wastes can be used both as alternative raw materials and alternative fuels, which can remarkably benefit for the sustainable development of cement industry. However, the solid wastes often contain much toxic components, especially heavy metals such as Pb, Cd, Cu, Zn, etc., making it difficult to estimate the risk of the co-processing. Therefore, behaviors of these heavy metals in cement kiln are being intensively investigated by researchers enrolled in the co-processing techniques. At present, most studies are focused on incorporation of heavy metals in cement clinkers, including the amount and limitation of the incorporation and the speciations of the elements involved, while their volatilization during the co-processing is hardly investigated. In this study, sewage sludge(typical municipal solid waste) and Pb-Zn smelting slag(high toxic, hard to dispose) were chosen as the objective wastes to be co-processed. Influences of these wastes on cement clinkering were characterized, volatilization and incorporation of heavy mental components(Pb, Cd, Cu, Zn) in cement clinkering processing were studied as well. The results are supposed to be indicative for safe co-processing of wastes in cement kiln and for evaluating its environmental impacts. Major conclusions are drawn as follows:(1) Effects of the sewage sludge and Pb-Zn slag on cement clinkering and property. When sewage sludge was added, the decomposing temperature of calcite was lowered, while the clinkering temperature of cement clinker was increased. In addition, the total amount of heat evolution during cement hydration was decreased and the exothermic peak of heat evolution was retarded.With increased addition of the sewage sludge, amount of Belite and interstitial phases in the clinkers increased whereas that of Alite decreased and the crystal size of Alite was enlarged with irregular shape and severe surface erosion. In case of Pb-Zn slag, when its addition was less than 6.2%, the liquid phase appeared at lower temperature, favoring the solid phase reaction and sintering of clinkers. Besides, the inclusions in Alite decreased while most surfaces of the Alite/Belite crystals melted. The occurrence of exothermic peak of heat evolution was slightly retarded, while the total amount of heat evolution was almost unchanged.(2)Effects of burning temperature on volatilization of heavy metals. For highly volatile elements(Pb and Cd), the volatilizing ratio was increased with the rise of burning temperature, being above 80% at 1450℃. For non-volatile elements(Cu and Zn), the volatilizing ratio increased with the rise of burning temperature at 600 ~1300℃, while at 1300 ~1450℃, the liquid phase rapidly increased so that most Cu and Zn were incorporated in clinker minerals by forming non-volatile compounds. Thus the ratio of volatilized Cu and Zn was all less than 30% at 1450℃.(3) Effects of wastes addition on volatilization of heavy metals.When sewage sludge addition was less than 15%, volatilizing ratio of Cu and Zn increased. When sewage sludge addition was more than 15%, volatilizing ratio of Zn increased with slower rate until stable volatilizing ratio was achieved. In addition, volatilizing ratio of Cu gradually decreased, but Pb and Cd presented no regular behavior. In case of Pb-Zn slag, volatilizing ratio of Pb, Cd, Cu and Zn showed similar variation pattern,all of them increased first and then decreased with the growth of the Pb-Zn slag addition.(4) Effects of clinker modulus.Increasing KH(amount of Alite increased), decreasing SM(amount of liquid phases increased) and decreasing IM(amount of C4 AF increased and viscosity of liquid phases decreased) all contributed to the incorporation of heavy metals in clinkers with reduction of volatilization.(5) Speciations and distributions of heavy metals. Pb, Cd and Cu were mainly incorporated in silicate phases(1.85mg/g, 4.12mg/g, 6.14mg/g, respectively) while Zn was mainly incorporated in ferrite phases(25.61mg/g). In cement clinker, Ca2+ can be replaced by Pb2+ and Cd2+, forming silicate, aluminate and ferrite compounds.Based on these results, the suggested maximum addition of municipal sewage sludge and Pb-Zn slag during co-processing wastes in cement kiln were 10% and 2.5%, respectively. This work can benefit for further development of disposing wastes such as sewage sludge and Pb-Zn slag in cement kiln, which can also be indicative for the sustainable development of cement industry.